This invention relates to an optical glass having optical constants of refractive index (nd) within the range of 1.55-1.75 and Abbe number (.nu. d) within the range of 25-51, also having a large negative anomalous dispersion (.DELTA. .theta. g.F) and being substantially free of a lead compound and an arsenic compound.
As an optical glass having such optical constants and having a negative anomalous dispersion property, there is known a glass of a B.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --PbO system. This system of glass, however, requires a relatively large amount of PbO with the result that the glass tends to have a relatively high density and is insufficient in chemical durability and, for these reasons, the glass has only a limited application. As glass systems containing no PbO, there are known glasses disclosed in Japanese Patent Publication No. 50-18007, Japanese Patent Publication No. 58-49502 and German Patent Publication No. 4,032,566. These glasses require a large amount of polyvalent metal oxide or oxides such as Ta.sub.2 O.sub.5 or a large amount of ZrO.sub.2, Nb.sub.2 O.sub.5 and Ta.sub.2 O.sub.5 for imparting a negative anomalous dispersion property to the glasses and this poses the problem that it is difficult to melt a batch of raw materials completely to obtain a homogenious glass.
Development of an optical glass having large negative anomalous dispersion is quite useful for designing of various optical glasses. In a normal optical glass which does not have large anomalous dispersion, chromatic aberration can be removed for a two-color light by combining two types of optical lenses having different Abbe numbers. As regards light of other colors, however, there still exists a residual chromatic aberration which is expressed as secondary spectra. For correcting such secondary spectra, particularly light of the ultraviolet spectrum, two different optical lenses having extremely deviated positive and negative anomalous dispersion characteristics are combined together. By this arrangement, a substantial amount of the secondary spectra can be removed.
It is an object of the present invention to provide an optical glass having large negative anomalous dispersion which is equal to or greater than the prior art optical glasses and having an excellent homogeniety without containing compounds such as PbO and As.sub.2 O.sub.3 which are undesirable for the environment and without requiring the Ta.sub.2 O.sub.5 ingredient which is very costly.